A 22-year-old male student reported for evaluation of a persistent chest radiopacity, following the completion of six months of antitubercular (Category I) therapy under the Revised National Tuberculosis Control Program (RNTCP). Seven months prior to this visit, he had cough and chest pain for a week duration and a chest radiograph taken elsewhere showed an opacity in right upper lobe that was diagnosed as smear negative pulmonary tuberculosis (TB). He had never smoked and denied any history of allergy, asthma, or recurrent respiratory symptoms. A physical examination was unremarkable. Blood routine investigations and spirometry results were normal. A chest radiograph showed two rounded opacities that appeared bilobed with smooth margin in right mid lung field in parahilar location [Figure 1]. A high-resolution computed tomography (HRCT) chest scan delineated a branched tubular opacity with surrounding parenchymal hyperlucency [Figure 2]a that terminated in a club-like opacity projecting from the hilum [Figure 2]b and c, a feature characteristic of congenital bronchial atresia (CBA). Punctate calcification is seen within the opacity in soft-tissue window [Figure 2]d. The patient was advised for periodic follow-up in view of the asymptomatic presentation.

Figure 1: Chest radiograph showing two smoothly marginated rounded opacities that appear to have a common stalk (arrows) in right parahilar lung field

CBA is a rare developmental anomaly characterized by focal interruption of normal continuity of a lobar, segmental, or subsegmental bronchus during intrauterine life. Typically, this leads to distal mucus impaction forming a bronchocele or mucocele, and is often associated with regional lung hyperinflation. [1],[2] The true incidence or prevalence of CBA is unknown, as many cases remain undiagnosed. The precise mechanism underlying CBA is not understood. It is proposed that CBA results from an ischemic insult after the 16 th week of intrauterine life or abnormal separation of the bronchial bud from the bud remnant or both. [2],[3] Moreover, CBA has been described in association with several other congenital anomalies, such as congenital cystic adenomatoid malformation, unilateral renal agenesis, and pericardial defect, which are known to develop during weeks 4-6 of intrauterine life, hence it may develop much earlier than proposed. [4] Furthermore, the bronchial pattern distal to the atretic segment is reported to be entirely normal, therefore some suggest that it may not be a true developmental anomaly; rather it happens secondary to a traumatic event during fetal life. [3] We believe the timing of occurrence of the ischemic or traumatic event during intrauterine life is crucial in determining the generation of bronchus that is affected by atresia. The majority of people with CBA are asymptomatic and the diagnosis is usually incidental. Most cases are diagnosed in the second or third decade of life and a male predominance is noted. Reportedly, the left upper lobe is most commonly affected; [3] however eight out of 12 patients had right lung involvement in a recent series. [2]

The usual chest radiographic findings are tubular branching, round or oval opacity, mostly solitary with or without surrounding hyperlucency. Expiratory films are better in delineating the hyperlucency. [2],[3] A computed tomography (CT) scan is the most sensitive tool for its diagnosis. The characteristic CT features include presence of mucocele, lack of proximal communication of the mucocele, and peripheral lung hyperinflation. CT also helps in differentiating CBA from vascular anomaly by delineating non-enhancement of such lesions on contrast study. The mucocele is always a round or club-like area extending from the hilum, and in some cases, the mucocele may be located in a peripheral area. Distal hyperinflation is believed to be caused by collateral ventilation through intraalveolar pores of the Kohn, bronchoalveolar channels of Lambert, and interbronchiolar channels. [2],[3] Bronchoscopy may reveal blind-ending bronchus or may be normal in up to 50% cases. [2] Hence, CBA is largely a radiological diagnosis and bronchoscopy has supplementary role.

On a chest radiograph, CBA may be misdiagnosed as one of several causes of mucoid impaction. [5] Interestingly, the index case was diagnosed as pulmonary tuberculosis; however there was no response to a full course of antitubercular treatment. This might happen as CBA is a rare disorder, while TB is endemic in India and both diseases commonly affect the upper lobes. Currently, most cases of pulmonary TB in India are diagnosed under the RNTCP, following an algorithmic approach. [6] Smear negative pulmonary TB is a radiological diagnosis where two repeat sputum examinations are negative for acid-fast bacilli and chest radiograph is suspicious of TB. Whether the chest radiographic abnormality is suggestive or not of TB is exclusively at the physician's discretion, and correct interpretation is crucial for appropriate diagnosis. Therefore, it is possible that some non-tubercular pulmonary diseases may be misdiagnosed as smear negative pulmonary TB, though the proportion of such patients is likely to be very small considering the number of true smear negative pulmonary TB diagnosed in a high-burden country like India. However, misdiagnosis can be avoided if one is aware of the distinctive radiological features of CBA like mucocele and regional hyperinflation. Wherever doubt exists, a HRCT scan should be performed for confirmation before initiating empirical treatment.

The treatment of bronchial atresia remains debatable. Some advocate surgery in all patients with an intent for a definitive diagnosis where as others believe in conservative approach and follow-up with regular chest radiographs. Currently, surgery is recommended in those who have recurrent infections or significant encroachment of the hyperinflated area on the adjacent normal lung or a malignancy cannot be excluded with certainty. [2],[3] We believe a conservative approach is more sensible as majority of patients are young and asymptomatic in whom the risk of malignancy is negligible.